Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital data signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous inkjet, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while un-imaged droplets are caught and returned to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles. These small particles can be stabilized against flocculation and settling by the use of distinct dispersing agents such as surfactants, oligomers, or polymers, or they can be directly functionalized to provide a self-dispersing characteristic. In either case the carrier medium can be a liquid or a solid at room temperature. Commonly used carrier media include water, mixtures of water and organic co-solvents, and high boiling organic solvents such as hydrocarbons, esters, ketones, alcohols, and ethers.
Pigment-based inkjet inks are often preferred over dye-based inkjet inks because of the superior image stability typically observed with the pigment-based inks. Self-dispersed pigments in turn are often preferred over surfactant-dispersed, oligomer-dispersed or polymer-dispersed pigments because of their greater stability to a variety of ink formulations and environmental keeping conditions. Self-dispersed pigments are typically used when high density and sharp images are required such as for the printing of text and graphics, and are especially useful when printing on to plain papers (ie. papers not specifically designed to render photographic quality images).
Self-dispersed pigments useful for inkjet printing have been prepared by a number of different processes. U.S. Pat. Nos. 5,554,739; 5,803,959; and 5,922,118 disclose covalent functionalization of pigment surfaces using diazonium compounds. U.S. Pat. Nos. 5,609,671; 5,718,746; 6,099,632; and 7,232,480 describe anionic self-dispersed pigments prepared by a hypochlorite oxidation process. U.S. Pat. No. 6,852,156 describes anionic pigments prepared by ozone oxidation.
Among the different types of self-dispersed pigments, those having a high degree of surface functionalization provide advantages in the printing of inkjet images. US Patent Publication No. 2007/0028800 discloses self-dispersed pigments having a charge equivalence of at least 0.5 mEq/g that have been carboxylate functionalized. U.S. Pat. No. 5,861,447 and US Patent Publication No. 2008/0206465 disclose self-dispersed pigments having greater than 11 weight % volatile surface functional groups.
Although self-dispersed pigments have a number of advantages when used in inkjet inks, they also present disadvantages. For example, self-dispersed pigment inks are particularly susceptible to smearing, especially with respect to high-lighter markers used in the marking of text images. It known in the art of self-dispersed pigment inks to add water-soluble polymers, neutralized with organic or inorganic bases, to improve the smear resistance of the printed images. Typically, the amount of base used to neutralize the ionizable groups of the water-soluble polymers is maintained at a level less than or equal to 100% of the ionizable groups. The presence of polymers in the inks can present additional limitations in ink performance. The presence of significant amounts of polymers in a self-dispersed pigment ink can reduce the amount of achievable density in the printed image.
Inkjet printing systems impose additional requirements on the design of ink formulations in order to achieve reliable ejection performance from the inkjet printhead. Polymers present in the ink can cause severe degradation in jetting performance of the ink due to the interaction with printhead materials such as the heater surface in a thermal inkjet printer. One particularly challenging problem with inkjet printers is known in the art as latency. Latency refers to the conditions where ink ejection from the printer is in a state of idle or where time between successive ink ejections from the printhead nozzles is long. During the time interval where ink ejection is idle the volatile ink components such as water, undergo evaporation in and around the printhead nozzles. Under such conditions components normally solubilized within the water become less soluble in the remaining components, such as non-volatile humectants. Water-soluble polymers are particularly susceptible to the evaporation of water during latency events resulting in the formation of polymer deposits on the walls, nozzle bore, or surfaces of the ejector. These polymer deposits, herein referred to as nodules, can directly lead to ejection problems such as, for example, velocity losses and directionality issues that detract from the overall image quality of the printed image. In extreme cases, the polymer deposits prevent ejection of ink altogether thereby causing catastrophic failure of the printer.
There remains a need to provide an ink composition comprising self-dispersing pigments that can provide high print density and text sharpness when printed onto an ink receiving medium and which reduces polymer deposits on components of the printing system during periods of latency.